Ultrasonic hearing is a recognized auditory effect that allows humans to perceive sounds of a much higher frequency than would ordinarily be audible using the physical inner ear, usually by stimulation of the base of the cochlea through bone induction. Human hearing is recognized as having an upper bound around 17-20 kHz, depending on the person, but ultrasonic sinusoids as high as 120 kHz have been reported as successfully perceived.
Two competing theories are proposed to explain this effect. The first [1] asserts that ultrasonic sounds excite the inner hair cells of the cochlea basal turn, which are responsive to high frequency sounds. The second [2] proposed that ultrasonic signals resonate the brain and are modulated down to frequencies that the cochlea can then detect.
By modulating speech signals onto an ultrasonic carrier, intelligible speech has also been perceived with a high degree of clarity, especially in areas of high ambient noise. Deatherage [3] states that what humans experience as ultrasonic perception may have been a necessary precursor in the evolution of echolocation in marine mammals. During the last decade and a half, hearing aids based on that effect have been marketed. All to date use large and cumbersome external transducers awkwardly mounted on headbands. These aids also require substantial power, especially for those with severe hearing losses. As such, there is a present need for transducers which efficiently provide ultrasonic signals to the temporal bone.